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This article explains how and why to set your air conditioning or heating system blower fan to continuous operation in order to improve indoor air quality by increased and continuous indoor air filtration. (Naturally if there is an IAQ problem source in the building it is essential to also find and correct that condition.

This website answers almost any question you might ask about air filters for heating or air conditioning systems. Readers should also see our INDOOR AIR QUALITY IMPROVEMENT GUIDE article series.

© Copyright 2012 InspectAPedia.com, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use page top links to major topics or use links at the left of each page to navigate within topics and documents at this website. Green links show where you are in a document series or at this website.

Why Run an Air Conditioner or Heating Blower Fan Continuously?

For maximum IAQ improvement in buildings: for the new blower installation we selected a fan unit which had both the capability of delivering adequate CFM of air flow and a blower fan motor duty cycle which permits continuous operation if we wish to run the system that way. Running the blower continuously at low speed resulted in continual air scrubbing in the building. When the heating or cooling needs of the building require, the fan shifts automatically to high speed. Here are the details:

Here are some possible advantages of continuous blower fan operation

• Improved air filtration, lower indoor dust levels - this is particularly true if you have a good air filter system installed. In our forensic lab when we installed a new gas furnace we asked the installer to include a multi speed fan (the fan chooses the needed speed automatically), a fan rated for continuous duty cycle, and we specified that the air filter system include HEPA filtration as well as an electrostatic air cleaner. We also included a cheap disposable filter at the air returns.
  
  This fan plus filter design permitted us to run the blower fan continuously in our lab, regardless of whether or not the heat was running (we don't need central air). We measured airborne dust levels in the building before and after installation of the new system and found that indoor dust levels improved by at least one order of magnitude.
  
  This system was enormously more effective than the dust filtration capacity of any free-standing "air cleaner" found on the popular consumer product market. That's because a warm air central heating system or central air conditioning system blower and duct work move many more cubic feet of air per minute through the filter system than a free-standing room "air cleaner" or "air purifier" can handle.
  
  
• Increased building comfort: especially if a multi-speed or variable speed fan motor is included in the system design, the continuous movement of air in the building avoids sudden hot or cold blasts when heating or air conditioning turns on; indoor air temperatures tend to be more even.

Here are some disadvantages of continuous air conditioner or heating system blower fan operation

• Energy use: A downside of continuous blower fan operation is higher electrical costs. On the other hand, if we needed to reduce building dust and allergen levels and/or increase comfort levels, this approach might avoid wasting money on other attempts at air cleaning that are expensive and less effective. More careful study is needed to really evaluate the claim that continuous blower fan operation uses more overall energy. In some cases there may be a net energy savings, depending on how hard the heating or cooling system has to work to satisfy the thermostat (and occupants) during "on" cycles.
  
  
• Equipment life: if the blower motor is not rated for continuous duty operation, it may need replacement sooner when continuous fan operation is used. In this case be sure the new blower is rated for continuous duty.

How to Put the Air Conditioner or Heat Pump Blower Fan into Continuous-On Operation

Most heating or cooling systems have one or perhaps two methods for turning the blower fan to continuous "on": The thermostat may include a "Fan On" position - try moving the fan switch from "Auto" to "Fan On". See our photo at the top of this page. The fan limit switch located in the blower compartment may include a "Manual Fan" Switch. To find this manual fan switch, see the articles, photos, and sketches in these articles How to Test the Fan & Limit Control and see How to Install the Fan & Limit Control found in our article series about the FAN LIMIT SWITCH . See the white knob at lower left in our fan limit switch photo at left. The cover has been removed from the switch in this picture.

Air Filtration Suggestions

See these articles about how to use air filters, where to buy them, and how to turn on and off HVAC blower fans

• AIR FILTERS for HVAC SYSTEMS - about air filters
• AIR FILTERS, OPTIMUM INDOOR - optimum filter designs for best filtration
• CONTINUOUS BLOWER FAN OPERATION - advantages of continuous air filtration in improving indoor air quality
• AIR FILTER EFFECTIVENESS - how air filter effectiveness is measured
• FIBERGLASS & AIR FILTERS - questions and opinions about fiberglass shedding hazards from air filters
• SOURCES FOR AIR FILTERS - where to buy air filters
• OTHER AIR CLEANERS
• Other Switches on a Room Thermostat - how to set the HVAC blower fan to continuous operation, and what to do if the blower fan won't turn on or won't turn off
• OZONE AIR PURIFIER WARNINGS

Questions & Answers regarding this article

Questions & Answers about whether or not to run the heating or air conditioning blower fan continuously

Question: what is the relationship between continuous blower fan operation and indoor humidity where central air is installed? Can condensate in the air handler raise indoor humidity?

Edgardo:

A disadvantage I found running the AC fan continuously in my residence, was a noticeable increase in the humidity in the house. The fan would blow across the wet coils and drain pan while the compressor wasn't running and put the water it had removed from the air during the prior cooling cycle back into the living space. So I stopped doing that. - Edgardo

Reply: from DF

Edgardo, thanks for the comment, it's an opportunity to clear up some confusion.

When your compressor is not running the cooling coil in your air handler is not cool and NO condensate will be produced in the unit. So if you are seeing too much water in the air handler, including as you say, while the compressor is not running, then more likely your condensate drain system is not draining adequately and a reservoir of condensate is remaining in the air handler condensate drip tray.

You should not see standing water there ever. If you do, the drip tray is not draining and the tray or drain system need to be cleared and repaired. Details about condensate handling are in the article titled CONDENSATE HANDLING, A/C and details about target levels for indoor humidity are at HUMIDITY LEVEL TARGET.

Edgardo:

Hi Dan The pan drains fine, and the backup secondary pan is dry. I am not seeing any unusual amount of water. The problem I mentioned is caused simply by evaporation back into the living space of the residual condensation left on the coils and in the pan when the compressor shuts off. The parts can't be instantly dry when the compressor shuts off. Anyway I do run the fan more continuously in the winter, as between cycles the air will settle in the house and can be close to 50° at the floor when it's 75° 5 feet off the floor. Keeping the fan on keeps the air mixed up and I can run the heat a degree or two lower.

Dan:

Edgardo I'm surprised that the volume of water left on the cooling coil after the A/C system shuts off would provide a measurable quantity of total humidity in the living space. Consider the total volume of water that's resting on the coils (as you explain the drain pan is dry) - it has to be less than a quart. Evaporate a quart of water into the cubic feet of air in even a small home - it just would not be likely to explain an indoor humidity problem.

I agree that running the fan gives much better air distribution and more even temperatures (and RH) throughout the home.

If you are seeing high indoor RH I'd look at whether or not the system is dehumidifying adequately when it's running. Incidentally, as we explain at DEHUMIDIFICATION PROBLEMS, an oversized A/C unit will cool but won't dehumidify the space.

Try making some actual RH measurements throughout the A/C cycle and let us know what you're seeing. See Tools for Measuring Humidity.

Edgardo:

I mentioned before that the main pan drains fine, but it will still be wet until the residual evaporates, and the secondary pan is dry which is there in case the primary pan drain gets clogged. There should never be water in that secondary catch pan otherwise the main drain is clogged. Anyway, all that is normal. I have access to the condensate drain pipe, and yesterday afternoon I put it into a gallon jug and captured almost a half-gallon of condensate water in 1 hour.

Relative humidity in the house right now is 42%. Temperature is 78°. I have a 1953-built frame house that I think cost about $8,000 new when it was built. Needless to say it has lousy insulation and leaks air everywhere, and moisture up from the crawl space. So I think the AC does well all things considered. Oversized system? Don't think so. It can just keep up with the heat load, and cycle off once in a while when it's 100°+ outside. We have some pretty extreme summer weather and humidity conditions in Texas and the system handles it pretty well with reasonably low electricity usage.

Dan:

Edgardo I agree that if your RH is down to 42% that's a suggestion that the A/C is both cooling and dehumidifying and is not oversized. Perhaps taking some RH measurements at regular intervals and when the system is cycling on and off will give a more clear picture of the indoor RH.

It just doesn't seem likely to me that the volume of moisture residual in a wet but otherwise drained condensate pan would explain a measurable variation in indoor RH in the rest of the building. That volume of water evaporated into the volume of air in a building could not explain high indoor RH. It may be more likely that if the home is poorly insulated and more, drafty, indoor Rh climbs along with temperature between cooling cycles.

Also if the RH was coming from the ductwork as a prime source when the system is off, you'd find higher RH at the duct openings as well as evidence of airflow out of the ducts when the cooling system was off. But be sure to measure RH at both the return and supply sides and registers so we're not confused by cycling higher RH house air with moisture coming from other sources or outdoors. Some measurements ought to clear that up.

Please also be sure to take a look at DEHUMIDIFICATION PROBLEMS where we describe some sources of and cures for high indoor humidity in buildings.

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

AIR FILTERS for HVAC SYSTEMS
  Air Filter Accessibility
  Air Filter Bypass Leakage
  Air Filter Installation Procedure
  Build Large Air Filters
  Air Filter Location
  Dirty Air Filter Problems
  ENERGY SAVINGS by AIR FILTERS
  Air Filtering Strategies
  Missing or Leaky Air Filters
 AIR FILTERS, OPTIMUM INDOOR
  Air Filter Efficiency
  Optimum Air Filter Design / Location
  How to Cascade Air Filters
 CONTINUOUS BLOWER FAN OPERATION
 AIR FILTER EFFECTIVENESS
  MERV, HEPA Definitions
  MERV Air Filter Ratings
  How Air Filters Work
  Particle Sizes & IAQ
  Gasketed air filters
  Debris in Return Air Plenum
  Washable air filters
 AIR FILTERING STRATEGIES
 FIBERGLASS & AIR FILTERS
  Air Filters Shed Fibers?
  Fiberglass & Test Lab Accuracy
  Variations in Fiber Size
 SOURCES FOR AIR FILTERS
  OTHER AIR CLEANERS

• Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects" section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and home inspection educator.
• Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who provided photos of failing Goodman gray flex duct in a hot attic.
• Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
• Wikipedia provided background information about the definition of HEPA and airborne particle interception.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
• Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.

• Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
• Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
• "Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
• Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
• Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
• Air Conditioning Inspection, Diagnosis, Repair, Efficiency all the basics for home owners, inspectors, new repairmen
• Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard
• Asbestos HVAC Ducts and Flues field identification photos and guide
• Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
• ...